This invention relates to power controllers, specifically those for use in controlling the power applied to heating elements used in annealing of very large metal objects. In particular this invention is useful in the annealing of installed and irradiated nuclear reactor pressure vessels. More particularly, this invention relates to power controller devices which provide high resolution linear power adjustment of the power applied, thereby obviating the need for measurement of power delivered to the load by the power controller for control purposes. The load may be a heating element, for example.
Many types of power controller devices are known in the prior art. The simplest power controller, known for a century or more, uses a rheostat or a variable ratio transformer to control the delivered power. For simple annealing, such as small glass kilns, such devices suffice. This approach suffers the disadvantage, however, of having non-linear control. That is, the "operator" controls current or more usually voltage approximately linearly, but the power delivered is proportional to the square of the varied quantity, so that the control (i.e., the responsiveness of the power delivered to changes in current or voltage) is inherently quadratic, not linear. Sophisticated annealing tasks, such as the annealing of nuclear reactor pressure vessels, require the accuracy of linear control, i.e., a linear variation in the power delivered in response to an operator-controlled change in current or voltage.
The standard approach to achieving linear power control has been to use systems which turn the power on for a portion of a fixed duty cycle, typically one or two seconds. Although the delivered power is quadratic in response to changes in the current or voltage, the amplitude of the AC current of the delivered power is approximately constant over the duty cycle, and hence the average power delivered over the duty cycle is linear with the proportion of time the delivered power is "on." The response time of the change in temperature of heating elements and the heated object to be annealed are such that time-varying power levels by turning the current on and off over one or two seconds has negligible effect on the temperature achieved.
Unfortunately, however, there are practical problems in using conventional systems to control heating elements used in sophisticated annealing such as that of nuclear reactor pressure vessels. The prior art linear power controller, which has been in use until the present invention, produces large step increases in power, especially at low percentages of delivered power, in reponse to changes in the controller settings, even with linear input control, because it can only add or subtract an integral cycle of the 60 Hertz (Hz) power signal, thereby producing highly discontinuous changes in delivered power level.
Increasing the length of the duty cycle helps but does not fundamentally alleviate the problem. Measurement of power delivered to the object to be annealed is therefore required to assure that actual power levels obtained are known to an acceptable accuracy.
In prior art designs using the above-described standard approach, 60 Hz power to the load is controlled using an on or off system with a fixed duty cycle such that integral numbers of complete 60 Hz sine cycles (synchronized to the line frequency) are applied to the load during the "on" portion of the controller duty cycle and no power is delivered during the "off" portion. For example, as illustrated in FIG. 1, at an input command of 10% power with a one second duty cycle, the controller should output to the load six complete 60 Hz cycles of power (0.1 second) every 60 cycles (1.0 second). Consequently, at low percentages of maximum deliverable power there is a significant time during which no power is applied to the load (e.g., 54 cycles at 10% power).
As described above, a problem inherent in this type of control is that large relative changes in power delivered are inherent at low absolute levels of power because the current can only change by an integral number of cycles. The control is thus grossly discontinuous in this region of operation. This problem can be referred to as granularity of control.
For example, an increase in power using this type of controller from six cycles or 10% power to seven cycles or 16% power is actually a relative change of 17% (one cycle out of six) in power being delivered. At 50% power (see FIG. 1), the load waveform for a duty cycle of one second supplies power for 30 cycles and is then off for 30 cycles. Specialty applications such as nuclear reactor annealing require not only the linearity of control offered by duty cycle systems but also precision of change of power level of one in a thousand, i.e., a resolution of 0.1% in relation to the 100% power level. With conventional controllers it has been necessary to measure load power due to the uncertainty at this level, especially at low power percentages. The added instrumentation required for feedback from the measurement of delivered power adds complexity and expense to the power controller apparatus. These measurements are also complicated due to the burst or discontinuous nature of the applied waveform.
Some improvement in resolution at low duty cycles can be achieved by increasing the total length of the duty cycle, e.g., to two seconds. In this case, a 20% power level is provided by 12 cycles out of 120 cycles; the next step increase in power is to 13 cycles or approximately 10.8% power and therefore a change of only about 8% (one cycle out of 12). Although improved, this technique still has significant granularity and still requires measurement of load power for accuracy of control.
Thus there is a need for a simple and convenient power controller providing both linearity and high resolution in power control for meeting annealing specifications. In particular, reactor vessel annealing requires a well controlled, slowly changing, uniform heat distribution to preclude thermal stresses. The accuracy and the resolution of control of input power directly affects the uniformity of temperature which in turn directly controls the likelihood of a successful annealing. Uniformity is accomplished with multiple heater banks comprising a sufficient number of well controlled electrical heating elements. Step changes in electrical power input, and thus heat input, disrupt both the uniformity and the slow rate of change, potentially causing damaging thermal stresses.
In addition, the annealing of particular interest is for an installed, previously operated nuclear reactor vessel. Installed shielding materials are subject to heat damage at temperatures only slightly above minimum annealing temperatures. The margin between acceptable minimum annealing temperatures and maximum safe temperatures for these materials is very narrow. It is therefore extremely important that vessel temperature uniformity, and thus control of electrical input to the heating elements, be as precise as is practical to assure that the entire vessel has reached the necessary annealing temperature but also stays below maximum temperature constraints.
Two other factors which affect the degree of temperature control and uniformity are variations in line voltage and the tendency of heating elements to increase resistance with increasing temperature. It would be useful to be able to feed back to the control system for adjustment of the input control voltage a signal based on changes in the amplitude of the line voltage and another signal based on the temperature of the heating element to maintain stability and control. The granularity of the prior art control system makes use of such signals for fine control problematic.
When feedback signals are used, random noise in the input channel potentially subjects the system to a well known problem with feedback loops, oscillation. Thus there is also a need in systems using feedback signals to be able to minimize susceptibility to input noise. The current invention answers all of these needs.